a student determines the molar mass of a liquid unknown by the method used in th

Question

a student determines the molar mass of a liquid unknown by the method used in this experiment. She found that the equilibrium temperature of a mixture of ice and water was 1.0 degrees celcuis on her thermometer. When she added 11.1g of her unknown sample to the mixture, the temperature, after thorough stirring, fell to -3.0 degrees celcuis. She then poured off the solution through a screen into a beaker. The mass of the solution was 90.4g.

1. what is the molalilty of the unknown solution?

2.how much unknown liquid was in the decanted solution?

3. how much water was in the decanted solution?

4, what did she find the molar mass of the unknown liquid, assuming she made the calculation properly?

Explanation / Answer

1. we have 11.1 g of the unknown liquid in 90.4g of solution
mass of water is 90.4-11.1 = 79.3g
we have 11.1 g of unknown liquid in 79.3 g of water
mass in 1000 g = 11.1 x 1000 / 79.3 = 140 g / kg

2.11.1 g

3. 79.3 g

4. delta t = m x kf x 1 ; here we have 1 particle so i = 1; kf for water is 1.86

m = 4/ 1.86 = 2.151 moles of liquid / kg of water

so in 1 kg of water we have 2.410 moles = 140g

mass of 1 mole = 140 / 2.410 = 58.09 g / mole

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